Effect of Precursor Deficiency Induced Ca/P Ratio on Antibacterial and Osteoblast Adhesion Properties of Ag-Incorporated Hydroxyapatite: Reducing Ag Toxicity

Ag-containing hydroxyapatite (HA) can reduce risks associated with bacterial infections which may eventually require additional surgical operations to retrieve a failed implant. The biological properties of HA in such applications are strongly affected by its composition in terms of dopants as well as Ca/P stoichiometry, which can be easily controlled by altering processing parameters, such as precursor concentrations. The objective of this in vitro study was to understand the effect of variations in HA precursor solutions on antibacterial properties against Escherichia coli (E. coli) and for promoting osteoblast (bone-forming cell) adhesion on Ag incorporated HA (AgHA) which has not yet been investigated. For this, two groups of AgHAs were synthesized via a precipitation method by adjusting precursor reactants with a stoichiometric value of 1.67, being either (Ca + Ag)/P (Ca-deficient) or Ca/(P + Ag) (P-deficient), and were characterized by XRD, FTIR, and SEM-EDS. Results showed that Ag⁺ incorporated into the Ca²⁺ sites was associated with a corresponding OH⁻ vacancy. Additional incorporation of CO₃²⁻ into PO₄³⁻ sites occurred specifically for the P-deficient AgHAs. While antibacterial properties increased, osteoblast adhesion decreased with increasing Ag content for the Ca-deficient AgHAs, as anticipated. In contrast, significant antibacterial properties with good osteoblast behavior were observed on the P-deficient AgHAs even with a lower Ag content, owing to carbonated HA. Thus, this showed that by synthesizing AgHA using P-deficient precursors with carbonate substitution, one can keep the antibacterial properties of Ag in HA while reducing its toxic effect on osteoblasts.

Using HAADF-STEM for atomic-scale evaluation of incorporation of antibacterial Ag atoms in a β-tricalcium phosphate structure

Structural evaluation of ionic additions in calcium phosphates that enhance their performance is a long-lasting area of research in the field of biomedical materials. Ionic incorporation in β-tricalcium phosphate (β-TCP) structures is indispensable for obtaining desirable properties for specific functions and applications. Owing to its complex structure and beam-sensitive nature, determining the extent of ion incorporation and its corresponding location in the β-TCP structure is challenging. Further, very few experimental studies have been able to estimate the location of Ag atoms incorporated in a β-TCP structure while considering the associated changes in lattice parameters. Although the incorporation alters the lattice parameters, the alteration is not significant enough for estimating the location of the incorporated Ag atoms. Here, Ag incorporation in a β-TCP structure was evaluated on atomic scale using scanning transmission electron microscopy (STEM). To the best of our knowledge, this is the first report to unambiguously determine the location of the incorporated Ag atoms in the β-TCP structure by comparing z-contrast profiles of the Ag and Ca atoms by combining the state-of-art STEM observations and STEM image simulations. The Ag incorporation in the Ca(4) sites of β-TCP, as estimated by the Rietveld refinement, was in good agreement with the high-angle annular dark-field STEM observations and the simulations of the location of Ag atoms for [001] and [010] zone axes.